Among its many uses, tantalum powder is generally utilized to produce tantalum capacitors. Solid tantalum capacitors are typically manufactured by compressing tantalum powder to form a pellet, sintering the pellet in a furnace to form a porous body, and then subjecting the porous body to anodization in a suitable electrolyte to form a continuous dielectric oxide film throughout the internal and external surface of the sintered porous body. The pores in the anode thus formed are then filled with a electrolyte. Then the entire anode body with filled pores is sealed to form a capacitor.
The quality of tantalum capacitors is generally determined by rating the capacitance, voltage capability, and current leakage of the capacitor. These characteristics are determined by the tantalum powder and the capacitor manufacturing process.
The surface area of the tantalum powder is important for the production of high quality capacitor. High surface area tantalum powder may be utilized to make high surface area anodes. The anode capacitance at a given voltage is directly related to anode surface area. High surface area anodes are therefore desirable.
The purity of the tantalum powder is also important to the production of high quality capacitors. Metallic and non-metallic impurities in the tantalum powder degrade the dielectric oxide film formed during the production of the capacitor. The degradation of the dielectric oxide film causes high current leakage from the capacitor. Using a tantalum powder with reduced impurities results in less degradation of the dielectric oxide film and therefore less current leakage. Therefore, using a high purity tantalum powder to produce capacitors is advantageous.
Utilizing a high sintering temperature or a high anodizing voltage in the production of the capacitor tends to alleviate the current leakage problem caused by high powder impurities. However, both methods reduce the net surface area of the anode and therefore the capacitance of the capacitor.
Tantalum powders are generally produced via one of two methods: a mechanical method, or a chemical method. The mechanical method includes the steps of: electron beam melting of tantalum powder to form an ingot, hydrating the ingot, milling the hydrides, dehydriding, cleaning, and heat treating. This method generally produces powder with high purity, which is utilized in capacitor applications where high voltage or high reliability is required. The mechanical method, however, suffers from high production costs. Additionally, the tantalum powders produced by the mechanical method generally have low surface areas.
The other generally utilized method for producing tantalum powder is the chemical method. The chemical method involves the chemical reduction of a tantalum compound, with an active metal, generally referred to as "the reducing agent", and then the subsequent cleaning and heat treatment of the tantalum powder. Typical tantalum compounds include, but are not limited to, potassium fluorotantalate (K.sub.2 TaF.sub.7), sodium fluorotantalate (Na.sub.2 TaF.sub.7), tantalum chloride (TaCl.sub.5) and mixtures thereof. Generally the reducing agent is any metal capable of reducing the tantalum compound to tantalum metal including sodium, potassium, and mixtures thereof. The powder is sometimes further mechanically milled to enhance the surface area or porosity. Tantalum powders produced by the chemical method generally have surface areas higher than powders produced by the mechanical method. However, tantalum powders produced by the chemical method generally also have higher impurity levels than powders produced by the mechanical method.
In more detail, various techniques have been practiced for the production of tantalum powders by the chemical method. See for example, U.S. Pat. No. 4,067,736. A review of typical techniques is also set forth in the background section of U.S. Pat. No. 4,684,399, assigned to Cabot Corporation and the disclosure of which is hereby incorporated by reference.
Potassium fluorotantalate (K.sub.2 TaF.sub.7), a tantalum salt, can be electrolytically reduced to tantalum in a molten bath with diluent chloride and fluoride salts of sodium and potassium. Production rate is limited to the electrolysis parameters of current and voltage. Since the concentration gradients established prevent obtaining a high yield, the production rate is relatively low. The resulting tantalum powders tend to be coarse and dendritic, and produce anodes for electrolytic capacitors having very low capacitive charge. Considerable impurities are transferred to the product due to the galvanic corrosive activity on the reaction vessel components.
Tantalum powder also can be made by exothermic reaction in a inert atmosphere wherein the K.sub.2 TaF.sub.7 is mixed with a reducing agent. See example U.S. Pat. No. 4,231,790. The enclosed charge is indirectly heated until the exothermic reaction is spontaneously initiated. The ensuing uncontrollable reaction produces powders having a wide range of particle sizes. Although these powders have larger surface areas per unit weight than electrolytic powders, they must be classified extensively in order for them to be usable in the manufacture of anodes for electrolytic capacitors.
Commonly, tantalum powder is commercially produced by adding sodium to K.sub.2 TaF.sub.7 which has been previously dissolved in molten salt or molten diluent. In this reaction the K.sub.2 TaF.sub.7 and diluent salts are heated in a reaction vessel to a temperature above the melting point of the salt mixture. Liquid sodium then is added. The bath is held at essentially isothermal conditions while being stirred by an internal agitator. The resulting powder has a wide range of particles sizes. In order for these materials to be acceptable for the manufacture of anodes for electrolytic capacitors, they may require extensive classification to obtain the desired particle size distributions. The capacitive charge that can be obtained from anodes derived from these powders typically is in the intermediate range; greater than a low range of less than 7000 cv/g, and generally not as high as an upper range of greater than 15000 cv/g.
A modification of this stirred liquid phase reaction scheme involves the introduction of diluent salts to the stirred reaction bath. The addition of diluents such as NaCl and KCl to K.sub.2 TaF.sub.7 allows the use of lower bath temperatures. However, this modified process results in agglomerates of finely divided material, a tendency to pick-up impurities, and the production of excessive fines.
In another method, solid diluent salt and K.sub.2 TaF.sub.7 are mulled with liquid sodium and the mixture is heated to the point of initiating a spontaneous exothermic reaction. The exothermic reaction generated is not easily controlled and, therefore, the powder characteristics include varying particle sizes, broad particle size distributions, and varying electrical characteristics. These powders require classification to remove fine and coarse particles prior to their utilization in the manufacture of anodes for electrolytic capacitors.
As discussed above, the capacitance of a tantalum pellet is a direct function of the surface area of the sintered powder. Greater surface area can be achieved, of course, by increasing the grams of powder per pellet. Cost considerations however, have dictated that development be focused on means to increase the surface area per gram of powder utilized. Since decreasing the particle size of the tantalum powder produces more surface area per unit of weight, effort has been extended into ways of making the tantalum particles smaller without introducing other adverse characteristics that often accompany size reduction.
Various tantalum powder process techniques have been practiced which attempt to maximize the production of a powder having a select, small desired particle size and, therefore, increased surface area. For example, U.S. Pat. No. 4,149,876 teaches techniques for controlling particle size of tantalum powder product in a reduction process wherein liquid sodium is added to a molten bath of K.sub.2 TaF.sub.7 and a diluent salt. Sodium metal is added at an elevated rate until the reduction temperature is reached. It is reported that the rate of sodium injection (feed rate into the reactor) has an inverse effect on the particle size of the finished product. Critical to maintaining temperature control for high sodium injection rates is the ability to extract heat by means of forced cooling of the reaction mass in the reaction vessel. Use of forced cooling is reported to significantly reduce the overall process time and further reduced the particle size for the powder produced.
Another factor which contributes to forming high surface area tantalum powders is the use of large amounts of diluents, such as sodium chloride, in the reduction reaction. These diluents may also serve as an internal heat absorber for the system.
A further factor important to producing a fine particle size, high surface area tantalum powder, is the temperature at which sodium is injected into the molten bath. Lower temperatures facilitate fine particle size formation.
Another important factor in the control of particle size is the temperature of reduction. As disclosed, temperatures from about 760.degree. to about 850.degree. C. tended to produce smaller particles, while the temperatures from about 850.degree. to about 1000.degree. C. tend to produce somewhat larger particles.
According to U.S. Pat. No. 4,149,876, it-is particularly advantageous to use the above-described techniques, in combination (large amounts of diluent salt, low initial molten bath temperature, very fast sodium feed rate, and use of forced cooling to maintain constant temperature during the growth period), to produce a uniform, fine particle size, high surface area tantalum powder.
In all the previous reaction schemes outlined above, wherein tantalum powder is produced by reducing a tantalum compound with a reducing metal, the reactants are either mixed together and then heated in a closed vessel until an exothermic reaction is spontaneously initiated, or, a molten bath of the tantalum compound is maintained and reducing metal is fed into the bath so as to reduce the tantalum compound to tantalum powder.
In Japanese Patent Disclosure Sho 38-8 (1963), it was shown that a tantalum metal product suitable for metallurgical purposes could be made by a method wherein K.sub.2 TaF.sub.7 crystals, heated to a temperature below about 500.degree. C. were gradually dropped into a bath of sodium maintained at a temperature near its boiling point.
A later Japanese Patent Disclosure Sho 43-25910 (1968) reviewed the Sho 38-8 disclosure and stated that while the earlier reference disclosed a method for producing a tantalum product featuring purity favorable for metallurgical utility, such a product having a particle size range from less than 5 microns to more than 100 microns would be unsuitable for capacitor applications. This later reference then discloses a modification of the earlier method wherein molten K.sub.2 TaF.sub.7, including diluents, is added slowly to a stirred liquid sodium bath. A tantalum powder of between 5 microns and 100 microns, having a specific surface area less than about 750 cm.sup.2 /g, is produced. However, while this reference defines this product as being capacitor grade tantalum powder, by current standards, this powder would feature unacceptably low capacitance for capacitor utility.
U.S. Pat. No. 4,684,399 discloses a process for producing tantalum powder wherein a tantalum compound is added in a continuous or incremental manner to a reactor during the course of the reaction with a reducing metal. The rate of continuous addition or the amount of each increment can be varied depending on the particular tantalum powder product characteristics desired. Continuous addition, or the addition of smaller increments tend to favor increased capacitance. The addition of the reducing agent as a single unitary charge prior to the introduction of the tantalum compound, or alternatively, in a continuous or semi-continuous manner is also disclosed.
Hereafter, the term "continuous" addition refers to a non-interrupted period of addition of tantalum compound or reducing agent. "Semi-continuous" addition refers to a constant rate of addition of reducing agent which is interrupted.
It is also known to utilize the above described methods with various dopants to increase the yield of fine particle sizes. U.S. Pat. Nos. 3,825,802 and 4,009,007 disclose the use of phosphorous as a means for improving the electrostatic capacity of capacitors and the flow properties of tantalum powder. U.S. Pat. No. 4,582,530 discloses the addition of sulfur as a doping agent. The use of boron and other dopants are also known to those skilled in the art
As previously discussed, an important consideration in the production of tantalum powder for use in capacitors is the purity level. Impurities generally adversely affect the performance of capacitors. The impurities commonly appearing in tantalum powder may be generally categorized as lighter impurities and heavier impurities. The lighter impurities include carbon, calcium, and aluminum which generally come from water used to wash the powder; fluorine, chlorine, sodium, and potassium, which are from the reaction mass; nitrogen and hydrogen, which may form when the tantalum powder contacts air or water; and silicon, which comes from the potassium salt and sometimes from water. Generally, most of the lighter impurities evaporate during sintering and therefore the lighter impurities do not significantly adversely affect capacitor performance.
The heavier impurities include Fe, Ni, Cr, Mo, Co and other metals. In contrast to the lighter impuritie, the heavier impurities generally remain in the powder even after high temperature sintering. Therefore, unless the level of heavier impurities in the tantalum powder is reduced during the formation of the tantalum powder, the heavier impurities will remain and adversely affect capacitor performance.
Generally the source of the heavier impurities is the equipment utilized in the reduction step of the process for producing tantalum powder. This equipment includes the reduction cell, lid, and agitator which have contact with the reaction mass. The equipment is generally made of nickel, iron, or alloys which are easily attacked by the reaction components under reaction conditions.
According to one theory, heavier impurities are produced by processes wherein a thin film of metal oxide is formed on the metal surface of the reactor and the film is dissolved to form metal ions which are incorporated into the tantalum powder matrix during the powder formation process. The thin film of metal oxide may be formed by residual air in the reactor attacking the metal surface of the reactor. At process temperatures of about 80.degree. C. or above, metal oxides form more rapidly. Alternatively, water absorbed by the diluent salts or the tantalum compound, is released at temperatures above about 80.degree. C. and attacks the metal surface of the reactor to form a thin film of metal oxide. When the diluent salts or the potassium salt reach the molten state through reaction heat or external heating, the thin metal oxide film dissolves in the molten mass to form metal ions.
It would be advantageous to produce a high surface area, high purity tantalum powder by preventing the formation of the metal oxide film on metal surfaces of the reactor and thereby remove the source of the heavier impurities during the production of tantalum powder.